A Seismic Refraction Survey in the Northern Rocky Mountains: More Evidence for an Intermediate Crustal Layer

The results of two seismic profiles in the north-western United States indicate a crustal thickness in the northern Rocky Mountains (British Columbia and the Idaho-Montana border) of about 37 km. Rapidly diminishing amplitudes of the basement arrivals were interpreted as being the result of the velocity decreasing with depth. A strong second arrival phase was interpreted as arising from a velocity gradient at the top of the intermediate crustal layer. At a distance of 700 km a sharp drop in the amplitude of the first arrival phase was observed which may be the result of the penetration of the rays into a low velocity zone beginning at a depth of about 44 km.